Fix QuantumCircuit.compose for front=True and individual qargs

qiskit/circuit/quantumcircuit.py

@@ -2412,9 +2412,7 @@ def map_vars(op):
             new_clbits=mapped_clbits,
         )
         if append_existing:
-            dest._current_scope().extend(
-                append_existing, qubits=mapped_qubits, clbits=mapped_clbits
-            )
+            dest._current_scope().extend(append_existing)
 
         return None if inplace else dest
 

releasenotes/notes/fix-unwanted-remapping-in-circuits-compose-fb2e4dd2d082b93d.yaml

@@ -0,0 +1,7 @@
+fixes:
+  - |
+    Fixed :meth:`.QuantumCircuit.compose` raising :exc:`ValueError` (``'qubits'
+    argument is the wrong length``) when ``front=True`` is used with an explicit
+    ``qubits`` mapping.
+
+    See `#15834 <https://github.com/Qiskit/qiskit/issues/15834>`__.

test/python/circuit/test_compose.py

@@ -550,6 +550,23 @@ def test_compose_front_gate(self):
 
         self.assertEqual(output, expected)
 
+    def test_compose_front_smaller_circuit(self):
+        """Test composing a smaller circuit at the front with explicit qubits mapping."""
+
+        qc_base = QuantumCircuit(2)
+        qc_base.h(0)
+
+        qc_x = QuantumCircuit(1)
+        qc_x.x(0)
+
+        output = qc_base.compose(qc_x, qubits=[0], front=True)
+
+        expected = QuantumCircuit(2)
+        expected.x(0)
+        expected.h(0)
+
+        self.assertEqual(output, expected)
+
     def test_compose_adds_parameters(self):
         """Test the composed circuit contains all parameters."""
         a, b = Parameter("a"), Parameter("b")